This invention relates to an optical sensor for converting a distance value to a quantity of optical energy while using laser light of a predetermined wavelength and for converting said quantity of optical energy to an electrical value. Also, this invention relates to an apparatus for scanning the surface of a measured body by means of said sensor.
A sensor of the type mentioned above has been disclosed, for instance by J. Wessel in his publication "Surface Enhanced Optical Microscopy" in J. Opt. Soc. Am. B 2/9 (1985) 1538-1540. In operation, this known sensor is making use of surface plasmons or more generally of polaritons, which have been defined for instance by H. Raether in Spring Tracts in Modern Physics 88 (1980) 165. The principle of operation of this known sensor is based on the modification of Raman scattering by effects which have been discussed for instance by J. Gersten and A. Nitzan in their publication "Electromagnetic Theory of Enhanced Raman Scattering by Molecules Adsorbed on Rough Surfaces" in J. Chem. Phys. 73/7 (1980) 3023-3037, or also by R. Ruppin in his publication "Electric Field Enhancement Near a Surface Bump" in Solid State Comm. 39/8 (1981) 903-906, P.F. Liao and A. Wokaun in their publication "Lightning Rod Effect in Surface Enhanced Raman Scattering" in J. Chem. Phys. 76/1 (1982) 751-752, or A. Wokaun, J. P. Gordon and P. F. Liao in their publication "Radiation Damping in Surface-Enhanced Raman Scattering" in Phys. Rev. Lett. 48/14 (1982) 957-961.
Scanning apparatus for scanning the surface of a measured body by means of prior art sensors using electromagnetic effects has been disclosed, for instance by C. C. Williams and H. K. Wickramasinghe in their publication "Scanning Thermal Profiler" in appln. Phys. Lett. 49/23 (1986) 1587-1589.
Further related literature is, for instance "A New Geometry for Field Enhancement in Surface-Enhanced Spectroscopy" by P. K. Aravind, R. W. Rendell and H. Metiu in Chem. Phys. Lett. 85/4 (1982) 396-403, "Surface Shape Resonances" by P. C. Das and J. I, Gersten in Phys. Rev. 25/10 (1982) 6281-6290, and "Surface Modes and Optical Absorption of a Small Sphere Above a Substrate" by R. Ruppin in Surface Science 127 (1983) 108-118.
It has also been disclosed in EP-A-0112402 and in EP-A-0185782 to partly coat a light waveguide with a metal thin film and to use said light waveguide in a near-field optical application. However, said metal coating has never been provided on the tip of said light waveguide: nay, in EP-A-0185782 it is explicitly disclosed to provide apertures on the waveguide in order not to cover its tip located next to an object to be inspected, while in EP-A-0112402 it is explicitly disclosed to plastically deform the metallization so as to expose a tiny aperture for the same purpose.
From this prior art it is known to provide an uninterrupted light path through successive dielectric media, from the sensor to the object to be inspected, and to prevent any metallic medium from being interposed on said light path or, if any metallic medium is interposed, to remove it. Clearly, it follows therefrom that the phenomenon put to use in said prior art is the Raman effect or, in some cases, the diffusion of light.
The main drawback of this prior art is that reflected light cannot readily be separated from scattered light, whether it is Raman scattered or diffused. This lack of a simple separation means or method entails a loss of sensitivity of the method as well as major experimental problems which arise when the principles which this prior art is based on are put to use. Indeed, although these principles have already been known for years, no efficient reduction to practice or workable application has been obtained up to now.